Using 3,[125-I]3',5-triiodo-L-thyronine ([125I]T3), the binding and uptake of thyroid hormone in cultured Swiss 3T3-4 and GH3 cells were characterized. The results showed saturable, reversible and stereospecific binding of T3 to both cell types. The results also showed that the uptake of T3 is receptor mediated. Using affinity labeling techniques, a protein with a molecular weight of 55,000 was specifically labeled in both cell types. The 55,000-dalton protein is postulated to be involved in mediating the uptake of T3 into cells. Using electron spin resonance, the dynamic interactions of thyroid hormones with liposomes derived from L-Alpha-dimyristoyl-phosphatidylcholine was studied. The rotational and lateral diffusing of L-thyroxine in liposomes was determined.